Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPP third generation partnership project
ARQ automatic repeat request
BPSK binary phase shift keying
DAS distributed antenna system
FEC forward error correction
H-ARQ hybrid-ARQ
LTE long term evolution
OFDMA orthogonal frequency division multiple access
OTD orthogonal transmit diversity
MBMS multimedia broadcast and multicast services
MIMO multiple input, multiple output
NACK negative acknowledge
QAM quadrature amplitude modulation
QPSK quadrature phase shift keying
SINR signal to interference noise ratio
STTD space time transmit diversity
DSTTD double space time transmit diversity
V-BLAST vertical Bell Labs layered space-time architecture
WIMAX worldwide interoperability for microwave access (IEEE 802.16)
In present and future mobile communication systems it is desirable to provide high data rate coverage for mobile terminals serviced by the communication systems. Typically, only those mobile terminals that are physically close to a base station can operate with a very high data rate, and to provide a high data rate coverage over a large geographical area a large number of base stations would be required. As the cost of implementing such a system can be prohibitive, research is being conducted on alternative techniques to provide wide area, high data rate service. One of the most promising, and currently heavily researched techniques, is to use relays or relay nodes to distribute the data rate more evenly in a cell served by a particular base station. FIG. 1 illustrates this principle for a layer two relay located at a street crossing (see R. Pabst, B. H. Walke, D. C. Schultz, P. Herhold, H. Yanikomeroglu, S. Mukherjee, H. Viswanathan, M. Lott, W. Zirwas, M. Dohler, H. Aghvami, D. D. Falconer, G. P. Fettweis, “Relay-Based Deployment Concepts for Wireless and Mobile Broadband Radio”, IEEE Communications Magazine, September 2004). The darker areas indicate the gain in data rate achieved by the relay. Note that only very close to the base station is the data rate approximately half of the original data rate, however the high data rate is more evenly distributed in the coverage area.
One problem that is presented is how best to integrate relays into a wireless communication system. In general, the performance gain realized by the use of relays when integrated in a wireless communication system should by maximized, while the interference caused by relays to the communication system should be minimized. One approach is to use cooperative relaying, i.e., instead of one relay, two or more relays are engaged in a single transmission and thus similar benefits to those realized in multi-antenna transmissions can be achieved. Moreover, due to the higher spatial separation, as compared to co-located antennas in traditional MIMO systems, the MIMO channel matrix is typically better conditioned.
In addition, future radio communication systems are envisioned to be very flexible and to support many different services at the same time. OFDMA systems offer a high flexibility and allow, for example, broadcast, multicast and unicast transmissions at the same time. OFDMA is proposed for use in many future radio systems, such as 3GPP LTE and WiMAX, and has been researched in the WINNER project. Cooperative relaying operations for broadcast/multicast have been previously proposed, but there is no efficient relay operation that would allow broadcast/multicast and optimized unicast transmission at the same time. FIG. 2 illustrates such a network.
Of interest to the exemplary embodiments of this invention that are described below is a network where the source (S) cannot necessarily communicate directly with the destination (D), and communicates with the destination through relays (R). FIG. 3 illustrates such a network. For example, in the downlink of a cellular network the source would be the base station and the destination the mobile terminal.
To illustrate the concept, assume that the source controls the operation of the relay. Thus it tells the relays at what time and at which OFDMA sub-channel(s) they are to transmit a received packet to the destination. This reflects also the current implementation of cooperative relaying in the WINNER project (see WINNER II Deliverable D3.5.2, “Assessment of relay based deployment concepts and detailed description of multi-hop capable RAN protocols as input for the concept group work”, June 2007).
Instead of an optimized unicast transmission based on closed loop MIMO schemes with feedback, the relays can, for example, use open loop MIMO schemes such as V-BLAST for their unicast transmission. Reference with regard to V-BLAST can be made to “V-BLAST: An Architecture for Realizing Very High Data Rates Over the Rich-Scattering Wireless Channel”, P. W. Wolniansky, G. J. Foschini, G. D. Golden, R. A. Valenzuela Bell Laboratories, Lucent Technologies, Crawford Hill Laboratory (1998). Reference can also be made to “Simplified Processing for High Spectral Efficiency Wireless Communication Employing Multi-Element Arrays”, G. J. Foschini, G. D. Golden, R. A. Valenzuela, P. W. Wolniansky, IEEE Journal on Selected Areas in Communications, Vol. 17, No. 11, November 1999, pgs. 1841-1852.
Most of the cooperative relaying schemes proposed in the literature can deal with the case 1 shown in FIG. 4, when one of the relays cannot decode the received packet. In the case of distributed space time block codes only the relay that successfully decoded the data will transmit and the destination is still able to decode the data.
However, the published literature does not consider open loop MIMO schemes such as V-BLAST or space-time codes, and possible optimizations in relay systems for retransmissions when the second hop transmission fails.
Typically for MBMS it is assumed that a multimedia data stream is sent to the users in a cell in a manner similar to a broadcast service such as television. However, in many cases (e.g., firmware update of a user terminal, delivery of a new application, downloading of multi-media content for later use) it would be desirable to have the possibility for reliable data transmission, that is, where the user terminal can request a retransmission. MBMS without feedback is typically designed to cover at least 95% of the service area and robust modulation and coding is used, which requires a large amount of resources. However, for reliable data transmissions an infrequent channel quality feedback can be requested from the user terminals and, if all of the MBMS users have a sufficiently high SINR, then open loop MIMO schemes such as V-BLAST or space-time coding can be utilized to improve the resource utilization. In this case it is desirable to optimize the retransmissions.
Exemplary of different cooperative relaying schemes proposed in the literature, an overview of the cooperative relaying schemes studied in WINNER will be provided. In the WINNER project several cooperative relaying schemes that were investigated are as follows.
A first approach is a fixed relaying scheme, where two relays decode the signal of two users (both relays use SDMA to decode the signal of the two users) and forward it cooperatively to the base station using a distributed space time block code. This approach is best suited for use on the uplink.
A second approach is a relay cyclic delay diversity scheme that increases the frequency selectivity of the channel. With this method only one parameter, the cyclic shift in the time domain, can be controlled for each relay. Thus, it is not possible to optimize broadcast/multicast and unicast transmissions in different OFDMA sub-channels at the same time.
A third approach is an adaptive decode and forward scheme, and its enhancement YARP. In this case the relays only forward the signal if they can decode the input signal correctly. This technique is not designed to optimize a unicast or a broadcast/multicast transmission, but is intended instead to provide additional diversity.
Several publications relate at least in part to ARQ and cooperative relaying. However, in these publications it is assumed that the destination receives the signals of the source and the relay.
J. N. Laneman, D. N. C. Tse, and G. W. Wornell, “Cooperative Diversity in Wireless Networks: Efficient Protocols and Outage Behaviour”, IEEE Transactions on Information Theory, April 2003, describe a technique where the relay only transmits when necessary.
E. Zimmermann, P. Herhold, and G. Fettweis, “A Novel Protocol for Cooperative Diversity in Wireless Networks”, in The Fifth European Wireless Conference—Mobile and Wireless Systems beyond 3G, February 2004, describe a technique where the relay only transmits if it has decoded the data correctly.
J. N. Laneman, “Limiting Analysis of Outage Probabilities for Diversity Schemes in Fading Channels”, in IEEE Global Communications Conference (GLOBECOM), San Francisco, December 2003, describe a technique where the relay does not simply use repetition coding, but instead uses a different and jointly designed codeword.
E. Zimmermann, P. Herhold, P., and G. Fettweis, “The impact of cooperation on diversity-exploiting protocols”, 59th IEEE VTC 2004-Spring, 17-19 May 2004 Page(s): 410-414 Vol. 1A, describe a technique that is a combination of the preceding three approaches.
I. Stanojev, O. Simeone and Y. Bar-Ness, “Performance Analysis of Collaborative Hybrid-ARQ Protocols over Fading Channels”, in Proc. Sarnoff Symposium, NJ, USA, March 2006, describe the use of orthogonal space-time block coded H-ARQ.
None of the foregoing approaches, nor any others that the inventors are currently aware of, permit the simultaneous use of broadcast/multicast and optimized unicast transmission with cooperative relays.